1. Field of the Invention
The present invention relates to a beam light scanning apparatus for generating a beam light for scan, an image forming apparatus equipped with the beam light scanning apparatus, and a method for controlling generation of the beam light. More particularly, the present invention relates to a beam light scanning apparatus, an image forming apparatus, and a method for controlling generation of a beam light, which are suitable for, e.g., a copying machine and in which a transfer path of data corresponding to image information in each line is made up of plural channels extending until a driver for driving a light emitting unit that emits the beam light.
2. Description of the Related Art
Recently, various types of image forming apparatuses, such as digital copying machines and laser printers, have been developed and put already into practice in which an image is formed with a combination of scanned exposure using a laser beam light (hereinafter referred to simply as a “beam light”) and an electrophotographic process.
That type of image forming apparatus operates based on the principle that the surface of a single photoconductor drum is scanned and exposed at the same time using the beam light to form a single electrostatic latent image on the surface of the photoconductor drum, and the electrostatic latent image is transferred to a sheet of paper.
In the field of that type of image forming apparatus, there has recently been a demand for, in particular, a speedup of image formation. To meet such a demand, Patent Document 1; Japanese Unexamined Patent Application Publication No. 2001-091872, for example, proposes an apparatus in which a plurality of laser oscillators are disposed and the surface of a photoconductor drum is scanned and exposed at the same time using the plurality of laser oscillators, to thereby form a latent image for each of lines in the direction of main scan (i.e., the axial direction of a rotary shaft of the photoconductor drum). However, using the plurality of laser oscillators is disadvantageous in making design complicated and increasing the size of an apparatus structure itself. Also, because of the necessity of accurately controlling scan intervals of beam lights emitted from the plurality of laser oscillators in the direction of sub-scan (i.e., the direction in which the drum surface is moved with the rotation and which is perpendicular to the direction of main scan on the surface of the photoconductor drum), a control process is complicated and a processing load is so increased as to impede realization of the speedup.
To solve the above-mentioned problem, Patent Document 2; Japanese Patent Application No. 2004-168425 proposes an apparatus in which a single laser oscillator is provided, while an image data transfer path extending until the laser oscillator is provided in plural. More specifically, the proposed apparatus includes an image processing unit for receiving image data supplied from a scanner unit, executing predetermined image processing on the received image data, and outputting the processed digital image data for each of lines while dividing the data into two channels, i.e., image data trains of odd-numbered lines and image data trains of even-numbered lines. The image processing unit is associated with two channels of serial circuits made up of PWM's for processing the image data trains of the respective channels and laser drivers. Output terminals of the laser drivers in the respective channels are connected, for example, to the single laser oscillator via a wired OR circuit. With such an arrangement, the single laser oscillator can be driven with data transfer via the two lines, and the above-mentioned disadvantage of Patent Document 1 can be avoided.
Patent Document 2 also discloses, as a modification of the image data transfer circuit of plural channels, a circuit using a single laser driver wherein two PWM's for respectively PWM-modulating image data obtained from odd-numbered pixels and image data obtained from even-numbered pixels in parallel are disposed on the output side of the image processing unit, outputs of the two PWM's are combined with each other via an OR circuit, and the single laser driver is connected to an output of the OR circuit.
However, the arrangement disclosed in Patent Document 2 still has a drawback to be overcome. The drawback is caused by variations in performance attributable to the individual difference between the PWM for the odd-numbered pixel trains and the PWM for the even-numbered pixel trains.
That drawback will be described below with reference to FIGS. 7 and 8. In the following description, the PWM for the odd-numbered pixel trains is called PWM1 and the PWM for the even-numbered pixel trains is called PWM2.
FIG. 7A represents an example in which an output (pulse width: t1) of PWM1 and an output (pulse width: t2) of PWM2 are combined to produce a PWM output signal with a pulse width of t3=t1+t2. The outputs of PWM1 and PWM2 are issued after delays of t11 and t12, respectively, in sync with a common horizontal sync signal (BD signal). If there are no variations in the designated pulse widths, a combined output of t3=t1+t2 is produced.
On the other hand, FIG. 7B represents an example in which, because of an error caused in synchronization accuracy of PWM2, the output of PWM2 which should be issued after t12 is issued after t13. In this example, the combined output has a pulse width of t4 (t4<t3). Accordingly, a pulse width smaller than the desired value is resulted.
Further, FIG. 7C represents an example in which, because of an error caused in synchronization accuracy of PWM2, the output of PWM2 which should be issued after t12 is issued after t14. In this example, a combined pulse has a “split” and is separated into two pulses.
FIG. 8A similarly represents an example in which an output (pulse width: t1) of PWM1 and an output (pulse width: t2) of PWM2 are combined to produce a PWM output signal with a pulse width of t3=t1+t2. The outputs of PWM1 and PWM2 are issued after delays of t11 and t12, respectively, in sync with a common horizontal sync signal (BD signal). If there are no variations in the designated pulse widths, a combined output of t3=t1+t2 is produced.
Also, FIG. 8B represents an example in which, because of an error caused in pulse width of PWM1, the output of PWM1 which should be issued with the pulse width of t1 is issued with a pulse width of t5 (t5>t1). In this example, the combined output has a pulse width of t6=t1+t2 and the pulse width is not changed.
On the other hand, FIG. 8C represents an example in which, because of an error caused in pulse width of PWM1, the output of PWM1 which should be issued with the pulse width of t1 is issued with a pulse width of t8 (t8<t1). In this example, a combined pulse signal has a “split” and is separated into two pulses.
Thus, because of errors in synchronization accuracy or variations in pulse width between PWM1 and PWM2, the combined PWM output is apt to have a pulse width differing from the desired value or to cause a “split”, and an image is apt to show thickening or thinning of lines or to include white streaks. This results in deterioration of image quality.